/**
 * @file
 * This is the IPv4 packet segmentation and reassembly implementation.
 *
 */

/*
 * Copyright (c) 2001-2004 Swedish Institute of Computer Science.
 * All rights reserved. 
 * 
 * Redistribution and use in source and binary forms, with or without modification, 
 * are permitted provided that the following conditions are met:
 *
 * 1. Redistributions of source code must retain the above copyright notice,
 *    this list of conditions and the following disclaimer.
 * 2. Redistributions in binary form must reproduce the above copyright notice,
 *    this list of conditions and the following disclaimer in the documentation
 *    and/or other materials provided with the distribution.
 * 3. The name of the author may not be used to endorse or promote products
 *    derived from this software without specific prior written permission. 
 *
 * THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR IMPLIED 
 * WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF 
 * MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT 
 * SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, 
 * EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT 
 * OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS 
 * INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN 
 * CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING 
 * IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY 
 * OF SUCH DAMAGE.
 *
 * This file is part of the lwIP TCP/IP stack.
 * 
 * Author: Jani Monoses <jani@iv.ro> 
 *         Simon Goldschmidt
 * original reassembly code by Adam Dunkels <adam@sics.se>
 * 
 */

#include "lwip/opt.h"
#include "lwip/ip_frag.h"
#include "lwip/ip.h"
#include "lwip/inet.h"
#include "lwip/inet_chksum.h"
#include "lwip/netif.h"
#include "lwip/stats.h"
#include "lwip/icmp.h"

#include <string.h>

#if IP_REASSEMBLY
/**
 * The IP reassembly code currently has the following limitations:
 * - IP header options are not supported
 * - fragments must not overlap (e.g. due to different routes),
 *   currently, overlapping or duplicate fragments are thrown away
 *   if IP_REASS_CHECK_OVERLAP=1 (the default)!
 *
 * @todo: work with IP header options
 */

/** Setting this to 0, you can turn off checking the fragments for overlapping
 * regions. The code gets a little smaller. Only use this if you know that
 * overlapping won't occur on your network! */
#ifndef IP_REASS_CHECK_OVERLAP
#define IP_REASS_CHECK_OVERLAP 1
#endif /* IP_REASS_CHECK_OVERLAP */

/** Set to 0 to prevent freeing the oldest datagram when the reassembly buffer is
 * full (IP_REASS_MAX_PBUFS pbufs are enqueued). The code gets a little smaller.
 * Datagrams will be freed by timeout only. Especially useful when MEMP_NUM_REASSDATA
 * is set to 1, so one datagram can be reassembled at a time, only. */
#ifndef IP_REASS_FREE_OLDEST
#define IP_REASS_FREE_OLDEST 1
#endif /* IP_REASS_FREE_OLDEST */

#define IP_REASS_FLAG_LASTFRAG 0x01

/** This is a helper struct which holds the starting
 * offset and the ending offset of this fragment to
 * easily chain the fragments.
 */
struct ip_reass_helper
{
  struct pbuf *next_pbuf;
  u16_t start;
  u16_t end;
};

#define IP_ADDRESSES_AND_ID_MATCH(iphdrA, iphdrB)  \
  (ip_addr_cmp(&(iphdrA)->src, &(iphdrB)->src) && \
   ip_addr_cmp(&(iphdrA)->dest, &(iphdrB)->dest) && \
   IPH_ID(iphdrA) == IPH_ID(iphdrB)) ? 1 : 0

/* global variables */
static struct ip_reassdata *reassdatagrams;
static u16_t ip_reass_pbufcount;

/* function prototypes */
static void ip_reass_dequeue_datagram (struct ip_reassdata *ipr,
				       struct ip_reassdata *prev);
static int ip_reass_free_complete_datagram (struct ip_reassdata *ipr,
					    struct ip_reassdata *prev);

/**
 * Reassembly timer base function
 * for both NO_SYS == 0 and 1 (!).
 *
 * Should be called every 1000 msec (defined by IP_TMR_INTERVAL).
 */
void
ip_reass_tmr (void)
{
  struct ip_reassdata *r, *prev = NULL;

  r = reassdatagrams;
  while (r != NULL)
    {
      /* Decrement the timer. Once it reaches 0,
       * clean up the incomplete fragment assembly */
      if (r->timer > 0)
	{
	  r->timer--;
	  LWIP_DEBUGF (IP_REASS_DEBUG,
		       ("ip_reass_tmr: timer dec %" U16_F "\n",
			(u16_t) r->timer));
	  prev = r;
	  r = r->next;
	}
      else
	{
	  /* reassembly timed out */
	  struct ip_reassdata *tmp;
	  LWIP_DEBUGF (IP_REASS_DEBUG, ("ip_reass_tmr: timer timed out\n"));
	  tmp = r;
	  /* get the next pointer before freeing */
	  r = r->next;
	  /* free the helper struct and all enqueued pbufs */
	  ip_reass_free_complete_datagram (tmp, prev);
	}
    }
}

/**
 * Free a datagram (struct ip_reassdata) and all its pbufs.
 * Updates the total count of enqueued pbufs (ip_reass_pbufcount),
 * SNMP counters and sends an ICMP time exceeded packet.
 *
 * @param ipr datagram to free
 * @param prev the previous datagram in the linked list
 * @return the number of pbufs freed
 */
static int
ip_reass_free_complete_datagram (struct ip_reassdata *ipr,
				 struct ip_reassdata *prev)
{
  int pbufs_freed = 0;
  struct pbuf *p;
  struct ip_reass_helper *iprh;

  LWIP_ASSERT ("prev != ipr", prev != ipr);
  if (prev != NULL)
    {
      LWIP_ASSERT ("prev->next == ipr", prev->next == ipr);
    }

#if LWIP_ICMP
  iprh = (struct ip_reass_helper *) ipr->p->payload;
  if (iprh->start == 0)
    {
      /* The first fragment was received, send ICMP time exceeded. */
      /* First, de-queue the first pbuf from r->p. */
      p = ipr->p;
      ipr->p = iprh->next_pbuf;
      /* Then, copy the original header into it. */
      SMEMCPY (p->payload, &ipr->iphdr, IP_HLEN);
      icmp_time_exceeded (p, ICMP_TE_FRAG);
      pbufs_freed += pbuf_clen (p);
      pbuf_free (p);
    }
#endif /* LWIP_ICMP */

  /* First, free all received pbufs.  The individual pbufs need to be released 
     separately as they have not yet been chained */
  p = ipr->p;
  while (p != NULL)
    {
      struct pbuf *pcur;
      iprh = (struct ip_reass_helper *) p->payload;
      pcur = p;
      /* get the next pointer before freeing */
      p = iprh->next_pbuf;
      pbufs_freed += pbuf_clen (pcur);
      pbuf_free (pcur);
    }
  /* Then, unchain the struct ip_reassdata from the list and free it. */
  ip_reass_dequeue_datagram (ipr, prev);
  LWIP_ASSERT ("ip_reass_pbufcount >= clen",
	       ip_reass_pbufcount >= pbufs_freed);
  ip_reass_pbufcount -= pbufs_freed;

  return pbufs_freed;
}

#if IP_REASS_FREE_OLDEST
/**
 * Free the oldest datagram to make room for enqueueing new fragments.
 * The datagram 'fraghdr' belongs to is not freed!
 *
 * @param fraghdr IP header of the current fragment
 * @param pbufs_needed number of pbufs needed to enqueue
 *        (used for freeing other datagrams if not enough space)
 * @return the number of pbufs freed
 */
static int
ip_reass_remove_oldest_datagram (struct ip_hdr *fraghdr, int pbufs_needed)
{
  /* @todo Can't we simply remove the last datagram in the
   *       linked list behind reassdatagrams?
   */
  struct ip_reassdata *r, *oldest, *prev;
  int pbufs_freed = 0, pbufs_freed_current;
  int other_datagrams;

  /* Free datagrams until being allowed to enqueue 'pbufs_needed' pbufs,
   * but don't free the datagram that 'fraghdr' belongs to! */
  do
    {
      oldest = NULL;
      prev = NULL;
      other_datagrams = 0;
      r = reassdatagrams;
      while (r != NULL)
	{
	  if (!IP_ADDRESSES_AND_ID_MATCH (&r->iphdr, fraghdr))
	    {
	      /* Not the same datagram as fraghdr */
	      other_datagrams++;
	      if (oldest == NULL)
		{
		  oldest = r;
		}
	      else if (r->timer <= oldest->timer)
		{
		  /* older than the previous oldest */
		  oldest = r;
		}
	    }
	  if (r->next != NULL)
	    {
	      prev = r;
	    }
	  r = r->next;
	}
      if (oldest != NULL)
	{
	  pbufs_freed_current =
	    ip_reass_free_complete_datagram (oldest, prev);
	  pbufs_freed += pbufs_freed_current;
	}
    }
  while ((pbufs_freed < pbufs_needed) && (other_datagrams > 1));
  return pbufs_freed;
}
#endif /* IP_REASS_FREE_OLDEST */

/**
 * Enqueues a new fragment into the fragment queue
 * @param fraghdr points to the new fragments IP hdr
 * @param clen number of pbufs needed to enqueue (used for freeing other datagrams if not enough space)
 * @return A pointer to the queue location into which the fragment was enqueued
 */
static struct ip_reassdata *
ip_reass_enqueue_new_datagram (struct ip_hdr *fraghdr, int clen)
{
  struct ip_reassdata *ipr;
  /* No matching previous fragment found, allocate a new reassdata struct */
  ipr = memp_malloc (MEMP_REASSDATA);
  if (ipr == NULL)
    {
#if IP_REASS_FREE_OLDEST
      if (ip_reass_remove_oldest_datagram (fraghdr, clen) >= clen)
	{
	  ipr = memp_malloc (MEMP_REASSDATA);
	}
      if (ipr == NULL)
#endif /* IP_REASS_FREE_OLDEST */
	{
	  IPFRAG_STATS_INC (ip_frag.memerr);
	  LWIP_DEBUGF (IP_REASS_DEBUG,
		       ("Failed to alloc reassdata struct\n"));
	  return NULL;
	}
    }
  memset (ipr, 0, sizeof (struct ip_reassdata));
  ipr->timer = IP_REASS_MAXAGE;

  /* enqueue the new structure to the front of the list */
  ipr->next = reassdatagrams;
  reassdatagrams = ipr;
  /* copy the ip header for later tests and input */
  /* @todo: no ip options supported? */
  SMEMCPY (&(ipr->iphdr), fraghdr, IP_HLEN);
  return ipr;
}

/**
 * Dequeues a datagram from the datagram queue. Doesn't deallocate the pbufs.
 * @param ipr points to the queue entry to dequeue
 */
static void
ip_reass_dequeue_datagram (struct ip_reassdata *ipr,
			   struct ip_reassdata *prev)
{

  /* dequeue the reass struct  */
  if (reassdatagrams == ipr)
    {
      /* it was the first in the list */
      reassdatagrams = ipr->next;
    }
  else
    {
      /* it wasn't the first, so it must have a valid 'prev' */
      LWIP_ASSERT ("sanity check linked list", prev != NULL);
      prev->next = ipr->next;
    }

  /* now we can free the ip_reass struct */
  memp_free (MEMP_REASSDATA, ipr);
}

/**
 * Chain a new pbuf into the pbuf list that composes the datagram.  The pbuf list
 * will grow over time as  new pbufs are rx.
 * Also checks that the datagram passes basic continuity checks (if the last
 * fragment was received at least once).
 * @param root_p points to the 'root' pbuf for the current datagram being assembled.
 * @param new_p points to the pbuf for the current fragment
 * @return 0 if invalid, >0 otherwise
 */
static int
ip_reass_chain_frag_into_datagram_and_validate (struct ip_reassdata *ipr,
						struct pbuf *new_p)
{
  struct ip_reass_helper *iprh, *iprh_tmp, *iprh_prev = NULL;
  struct pbuf *q;
  u16_t offset, len;
  struct ip_hdr *fraghdr;
  int valid = 1;

  /* Extract length and fragment offset from current fragment */
  fraghdr = (struct ip_hdr *) new_p->payload;
  len = ntohs (IPH_LEN (fraghdr)) - IPH_HL (fraghdr) * 4;
  offset = (ntohs (IPH_OFFSET (fraghdr)) & IP_OFFMASK) * 8;

  /* overwrite the fragment's ip header from the pbuf with our helper struct,
   * and setup the embedded helper structure. */
  /* make sure the struct ip_reass_helper fits into the IP header */
  LWIP_ASSERT ("sizeof(struct ip_reass_helper) <= IP_HLEN",
	       sizeof (struct ip_reass_helper) <= IP_HLEN);
  iprh = (struct ip_reass_helper *) new_p->payload;
  iprh->next_pbuf = NULL;
  iprh->start = offset;
  iprh->end = offset + len;

  /* Iterate through until we either get to the end of the list (append),
   * or we find on with a larger offset (insert). */
  for (q = ipr->p; q != NULL;)
    {
      iprh_tmp = (struct ip_reass_helper *) q->payload;
      if (iprh->start < iprh_tmp->start)
	{
	  /* the new pbuf should be inserted before this */
	  iprh->next_pbuf = q;
	  if (iprh_prev != NULL)
	    {
	      /* not the fragment with the lowest offset */
#if IP_REASS_CHECK_OVERLAP
	      if ((iprh->start < iprh_prev->end)
		  || (iprh->end > iprh_tmp->start))
		{
		  /* fragment overlaps with previous or following, throw away */
		  goto freepbuf;
		}
#endif /* IP_REASS_CHECK_OVERLAP */
	      iprh_prev->next_pbuf = new_p;
	    }
	  else
	    {
	      /* fragment with the lowest offset */
	      ipr->p = new_p;
	    }
	  break;
	}
      else if (iprh->start == iprh_tmp->start)
	{
	  /* received the same datagram twice: no need to keep the datagram */
	  goto freepbuf;
#if IP_REASS_CHECK_OVERLAP
	}
      else if (iprh->start < iprh_tmp->end)
	{
	  /* overlap: no need to keep the new datagram */
	  goto freepbuf;
#endif /* IP_REASS_CHECK_OVERLAP */
	}
      else
	{
	  /* Check if the fragments received so far have no wholes. */
	  if (iprh_prev != NULL)
	    {
	      if (iprh_prev->end != iprh_tmp->start)
		{
		  /* There is a fragment missing between the current
		   * and the previous fragment */
		  valid = 0;
		}
	    }
	}
      q = iprh_tmp->next_pbuf;
      iprh_prev = iprh_tmp;
    }

  /* If q is NULL, then we made it to the end of the list. Determine what to do now */
  if (q == NULL)
    {
      if (iprh_prev != NULL)
	{
	  /* this is (for now), the fragment with the highest offset:
	   * chain it to the last fragment */
#if IP_REASS_CHECK_OVERLAP
	  LWIP_ASSERT ("check fragments don't overlap",
		       iprh_prev->end <= iprh->start);
#endif /* IP_REASS_CHECK_OVERLAP */
	  iprh_prev->next_pbuf = new_p;
	  if (iprh_prev->end != iprh->start)
	    {
	      valid = 0;
	    }
	}
      else
	{
#if IP_REASS_CHECK_OVERLAP
	  LWIP_ASSERT
	    ("no previous fragment, this must be the first fragment!",
	     ipr->p == NULL);
#endif /* IP_REASS_CHECK_OVERLAP */
	  /* this is the first fragment we ever received for this ip datagram */
	  ipr->p = new_p;
	}
    }

  /* At this point, the validation part begins: */
  /* If we already received the last fragment */
  if ((ipr->flags & IP_REASS_FLAG_LASTFRAG) != 0)
    {
      /* and had no wholes so far */
      if (valid)
	{
	  /* then check if the rest of the fragments is here */
	  /* Check if the queue starts with the first datagram */
	  if (((struct ip_reass_helper *) ipr->p->payload)->start != 0)
	    {
	      valid = 0;
	    }
	  else
	    {
	      /* and check that there are no wholes after this datagram */
	      iprh_prev = iprh;
	      q = iprh->next_pbuf;
	      while (q != NULL)
		{
		  iprh = (struct ip_reass_helper *) q->payload;
		  if (iprh_prev->end != iprh->start)
		    {
		      valid = 0;
		      break;
		    }
		  iprh_prev = iprh;
		  q = iprh->next_pbuf;
		}
	      /* if still valid, all fragments are received
	       * (because to the MF==0 already arrived */
	      if (valid)
		{
		  LWIP_ASSERT ("sanity check", ipr->p != NULL);
		  LWIP_ASSERT ("sanity check",
			       ((struct ip_reass_helper *) ipr->p->payload) !=
			       iprh);
		  LWIP_ASSERT ("validate_datagram:next_pbuf!=NULL",
			       iprh->next_pbuf == NULL);
		  LWIP_ASSERT ("validate_datagram:datagram end!=datagram len",
			       iprh->end == ipr->datagram_len);
		}
	    }
	}
      /* If valid is 0 here, there are some fragments missing in the middle
       * (since MF == 0 has already arrived). Such datagrams simply time out if
       * no more fragments are received... */
      return valid;
    }
  /* If we come here, not all fragments were received, yet! */
  return 0;			/* not yet valid! */
#if IP_REASS_CHECK_OVERLAP
freepbuf:
  ip_reass_pbufcount -= pbuf_clen (new_p);
  pbuf_free (new_p);
  return 0;
#endif /* IP_REASS_CHECK_OVERLAP */
}

/**
 * Reassembles incoming IP fragments into an IP datagram.
 *
 * @param p points to a pbuf chain of the fragment
 * @return NULL if reassembly is incomplete, ? otherwise
 */
struct pbuf *
ip_reass (struct pbuf *p)
{
  struct pbuf *r;
  struct ip_hdr *fraghdr;
  struct ip_reassdata *ipr;
  struct ip_reass_helper *iprh;
  u16_t offset, len;
  u8_t clen;
  struct ip_reassdata *ipr_prev = NULL;

  IPFRAG_STATS_INC (ip_frag.recv);

  fraghdr = (struct ip_hdr *) p->payload;

  if ((IPH_HL (fraghdr) * 4) != IP_HLEN)
    {
      LWIP_DEBUGF (IP_REASS_DEBUG,
		   ("ip_reass: IP options currently not supported!\n"));
      IPFRAG_STATS_INC (ip_frag.err);
      goto nullreturn;
    }

  offset = (ntohs (IPH_OFFSET (fraghdr)) & IP_OFFMASK) * 8;
  len = ntohs (IPH_LEN (fraghdr)) - IPH_HL (fraghdr) * 4;

  /* Check if we are allowed to enqueue more datagrams. */
  clen = pbuf_clen (p);
  if ((ip_reass_pbufcount + clen) > IP_REASS_MAX_PBUFS)
    {
#if IP_REASS_FREE_OLDEST
      if (!ip_reass_remove_oldest_datagram (fraghdr, clen) ||
	  ((ip_reass_pbufcount + clen) > IP_REASS_MAX_PBUFS))
#endif /* IP_REASS_FREE_OLDEST */
	{
	  /* No datagram could be freed and still too many pbufs enqueued */
	  LWIP_DEBUGF (IP_REASS_DEBUG,
		       ("ip_reass: Overflow condition: pbufct=%d, clen=%d, MAX=%d\n",
			ip_reass_pbufcount, clen, IP_REASS_MAX_PBUFS));
	  IPFRAG_STATS_INC (ip_frag.memerr);
	  /* @todo: send ICMP time exceeded here? */
	  /* drop this pbuf */
	  goto nullreturn;
	}
    }

  /* Look for the datagram the fragment belongs to in the current datagram queue,
   * remembering the previous in the queue for later dequeueing. */
  for (ipr = reassdatagrams; ipr != NULL; ipr = ipr->next)
    {
      /* Check if the incoming fragment matches the one currently present
         in the reassembly buffer. If so, we proceed with copying the
         fragment into the buffer. */
      if (IP_ADDRESSES_AND_ID_MATCH (&ipr->iphdr, fraghdr))
	{
	  LWIP_DEBUGF (IP_REASS_DEBUG,
		       ("ip_reass: matching previous fragment ID=%" X16_F
			"\n", ntohs (IPH_ID (fraghdr))));
	  IPFRAG_STATS_INC (ip_frag.cachehit);
	  break;
	}
      ipr_prev = ipr;
    }

  if (ipr == NULL)
    {
      /* Enqueue a new datagram into the datagram queue */
      ipr = ip_reass_enqueue_new_datagram (fraghdr, clen);
      /* Bail if unable to enqueue */
      if (ipr == NULL)
	{
	  goto nullreturn;
	}
    }
  else
    {
      if (((ntohs (IPH_OFFSET (fraghdr)) & IP_OFFMASK) == 0) &&
	  ((ntohs (IPH_OFFSET (&ipr->iphdr)) & IP_OFFMASK) != 0))
	{
	  /* ipr->iphdr is not the header from the first fragment, but fraghdr is
	   * -> copy fraghdr into ipr->iphdr since we want to have the header
	   * of the first fragment (for ICMP time exceeded and later, for copying
	   * all options, if supported)*/
	  SMEMCPY (&ipr->iphdr, fraghdr, IP_HLEN);
	}
    }
  /* Track the current number of pbufs current 'in-flight', in order to limit 
     the number of fragments that may be enqueued at any one time */
  ip_reass_pbufcount += clen;

  /* At this point, we have either created a new entry or pointing 
   * to an existing one */

  /* check for 'no more fragments', and update queue entry */
  if ((ntohs (IPH_OFFSET (fraghdr)) & IP_MF) == 0)
    {
      ipr->flags |= IP_REASS_FLAG_LASTFRAG;
      ipr->datagram_len = offset + len;
      LWIP_DEBUGF (IP_REASS_DEBUG,
		   ("ip_reass: last fragment seen, total len %" S16_F "\n",
		    ipr->datagram_len));
    }
  /* find the right place to insert this pbuf */
  /* @todo: trim pbufs if fragments are overlapping */
  if (ip_reass_chain_frag_into_datagram_and_validate (ipr, p))
    {
      /* the totally last fragment (flag more fragments = 0) was received at least
       * once AND all fragments are received */
      ipr->datagram_len += IP_HLEN;

      /* save the second pbuf before copying the header over the pointer */
      r = ((struct ip_reass_helper *) ipr->p->payload)->next_pbuf;

      /* copy the original ip header back to the first pbuf */
      fraghdr = (struct ip_hdr *) (ipr->p->payload);
      SMEMCPY (fraghdr, &ipr->iphdr, IP_HLEN);
      IPH_LEN_SET (fraghdr, htons (ipr->datagram_len));
      IPH_OFFSET_SET (fraghdr, 0);
      IPH_CHKSUM_SET (fraghdr, 0);
      /* @todo: do we need to set calculate the correct checksum? */
      IPH_CHKSUM_SET (fraghdr, inet_chksum (fraghdr, IP_HLEN));

      p = ipr->p;

      /* chain together the pbufs contained within the reass_data list. */
      while (r != NULL)
	{
	  iprh = (struct ip_reass_helper *) r->payload;

	  /* hide the ip header for every succeding fragment */
	  pbuf_header (r, -IP_HLEN);
	  pbuf_cat (p, r);
	  r = iprh->next_pbuf;
	}
      /* release the sources allocate for the fragment queue entry */
      ip_reass_dequeue_datagram (ipr, ipr_prev);

      /* and adjust the number of pbufs currently queued for reassembly. */
      ip_reass_pbufcount -= pbuf_clen (p);

      /* Return the pbuf chain */
      return p;
    }
  /* the datagram is not (yet?) reassembled completely */
  LWIP_DEBUGF (IP_REASS_DEBUG,
	       ("ip_reass_pbufcount: %d out\n", ip_reass_pbufcount));
  return NULL;

nullreturn:
  LWIP_DEBUGF (IP_REASS_DEBUG, ("ip_reass: nullreturn\n"));
  IPFRAG_STATS_INC (ip_frag.drop);
  pbuf_free (p);
  return NULL;
}
#endif /* IP_REASSEMBLY */

#if IP_FRAG
#if IP_FRAG_USES_STATIC_BUF
static u8_t buf[LWIP_MEM_ALIGN_SIZE (IP_FRAG_MAX_MTU)];
#endif /* IP_FRAG_USES_STATIC_BUF */

/**
 * Fragment an IP datagram if too large for the netif.
 *
 * Chop the datagram in MTU sized chunks and send them in order
 * by using a fixed size static memory buffer (PBUF_REF) or
 * point PBUF_REFs into p (depending on IP_FRAG_USES_STATIC_BUF).
 *
 * @param p ip packet to send
 * @param netif the netif on which to send
 * @param dest destination ip address to which to send
 *
 * @return ERR_OK if sent successfully, err_t otherwise
 */
err_t
ip_frag (struct pbuf *p, struct netif *netif, struct ip_addr *dest)
{
  struct pbuf *rambuf;
#if IP_FRAG_USES_STATIC_BUF
  struct pbuf *header;
#else
  struct pbuf *newpbuf;
  struct ip_hdr *original_iphdr;
#endif
  struct ip_hdr *iphdr;
  u16_t nfb;
  u16_t left, cop;
  u16_t mtu = netif->mtu;
  u16_t ofo, omf;
  u16_t last;
  u16_t poff = IP_HLEN;
  u16_t tmp;
#if !IP_FRAG_USES_STATIC_BUF
  u16_t newpbuflen = 0;
  u16_t left_to_copy;
#endif

  /* Get a RAM based MTU sized pbuf */
#if IP_FRAG_USES_STATIC_BUF
  /* When using a static buffer, we use a PBUF_REF, which we will
   * use to reference the packet (without link header).
   * Layer and length is irrelevant.
   */
  rambuf = pbuf_alloc (PBUF_LINK, 0, PBUF_REF);
  if (rambuf == NULL)
    {
      LWIP_DEBUGF (IP_REASS_DEBUG,
		   ("ip_frag: pbuf_alloc(PBUF_LINK, 0, PBUF_REF) failed\n"));
      return ERR_MEM;
    }
  rambuf->tot_len = rambuf->len = mtu;
  rambuf->payload = LWIP_MEM_ALIGN ((void *) buf);

  /* Copy the IP header in it */
  iphdr = rambuf->payload;
  SMEMCPY (iphdr, p->payload, IP_HLEN);
#else /* IP_FRAG_USES_STATIC_BUF */
  original_iphdr = p->payload;
  iphdr = original_iphdr;
#endif /* IP_FRAG_USES_STATIC_BUF */

  /* Save original offset */
  tmp = ntohs (IPH_OFFSET (iphdr));
  ofo = tmp & IP_OFFMASK;
  omf = tmp & IP_MF;

  left = p->tot_len - IP_HLEN;

  nfb = (mtu - IP_HLEN) / 8;

  while (left)
    {
      last = (left <= mtu - IP_HLEN);

      /* Set new offset and MF flag */
      tmp = omf | (IP_OFFMASK & (ofo));
      if (!last)
	tmp = tmp | IP_MF;

      /* Fill this fragment */
      cop = last ? left : nfb * 8;

#if IP_FRAG_USES_STATIC_BUF
      poff += pbuf_copy_partial (p, (u8_t *) iphdr + IP_HLEN, cop, poff);
#else /* IP_FRAG_USES_STATIC_BUF */
      /* When not using a static buffer, create a chain of pbufs.
       * The first will be a PBUF_RAM holding the link and IP header.
       * The rest will be PBUF_REFs mirroring the pbuf chain to be fragged,
       * but limited to the size of an mtu.
       */
      rambuf = pbuf_alloc (PBUF_LINK, IP_HLEN, PBUF_RAM);
      if (rambuf == NULL)
	{
	  return ERR_MEM;
	}
      LWIP_ASSERT ("this needs a pbuf in one piece!", (p->len >= (IP_HLEN)));
      SMEMCPY (rambuf->payload, original_iphdr, IP_HLEN);
      iphdr = rambuf->payload;

      /* Can just adjust p directly for needed offset. */
      p->payload = (u8_t *) p->payload + poff;
      p->len -= poff;

      left_to_copy = cop;
      while (left_to_copy)
	{
	  newpbuflen = (left_to_copy < p->len) ? left_to_copy : p->len;
	  /* Is this pbuf already empty? */
	  if (!newpbuflen)
	    {
	      p = p->next;
	      continue;
	    }
	  newpbuf = pbuf_alloc (PBUF_RAW, 0, PBUF_REF);
	  if (newpbuf == NULL)
	    {
	      pbuf_free (rambuf);
	      return ERR_MEM;
	    }
	  /* Mirror this pbuf, although we might not need all of it. */
	  newpbuf->payload = p->payload;
	  newpbuf->len = newpbuf->tot_len = newpbuflen;
	  /* Add it to end of rambuf's chain, but using pbuf_cat, not pbuf_chain
	   * so that it is removed when pbuf_dechain is later called on rambuf.
	   */
	  pbuf_cat (rambuf, newpbuf);
	  left_to_copy -= newpbuflen;
	  if (left_to_copy)
	    p = p->next;
	}
      poff = newpbuflen;
#endif /* IP_FRAG_USES_STATIC_BUF */

      /* Correct header */
      IPH_OFFSET_SET (iphdr, htons (tmp));
      IPH_LEN_SET (iphdr, htons (cop + IP_HLEN));
      IPH_CHKSUM_SET (iphdr, 0);
      IPH_CHKSUM_SET (iphdr, inet_chksum (iphdr, IP_HLEN));

#if IP_FRAG_USES_STATIC_BUF
      if (last)
	pbuf_realloc (rambuf, left + IP_HLEN);

      /* This part is ugly: we alloc a RAM based pbuf for 
       * the link level header for each chunk and then 
       * free it.A PBUF_ROM style pbuf for which pbuf_header
       * worked would make things simpler.
       */
      header = pbuf_alloc (PBUF_LINK, 0, PBUF_RAM);
      if (header != NULL)
	{
	  pbuf_chain (header, rambuf);
	  netif->output (netif, header, dest);
	  IPFRAG_STATS_INC (ip_frag.xmit);
	  pbuf_free (header);
	}
      else
	{
	  LWIP_DEBUGF (IP_REASS_DEBUG,
		       ("ip_frag: pbuf_alloc() for header failed\n"));
	  pbuf_free (rambuf);
	  return ERR_MEM;
	}
#else /* IP_FRAG_USES_STATIC_BUF */
      /* No need for separate header pbuf - we allowed room for it in rambuf
       * when allocated.
       */
      netif->output (netif, rambuf, dest);
      IPFRAG_STATS_INC (ip_frag.xmit);

      /* Unfortunately we can't reuse rambuf - the hardware may still be
       * using the buffer. Instead we free it (and the ensuing chain) and
       * recreate it next time round the loop. If we're lucky the hardware
       * will have already sent the packet, the free will really free, and
       * there will be zero memory penalty.
       */

      pbuf_free (rambuf);
#endif /* IP_FRAG_USES_STATIC_BUF */
      left -= cop;
      ofo += nfb;
    }
#if IP_FRAG_USES_STATIC_BUF
  pbuf_free (rambuf);
#endif /* IP_FRAG_USES_STATIC_BUF */
  return ERR_OK;
}
#endif /* IP_FRAG */
